1. Field of the Invention
This invention relates to a color television signal processing circuit, and, more particularly, to a clock signal reproducing circuit apparatus using for expanding the time scale of a compressed chrominance signal when a color television signal is reproduced from the color television signal, transmitted or recorded in such manner that the chrominance signal is compressed in time scale and is inserted in a time division manner during the horizontal blanking period of the luminance signal.
2. Description of the Prior Art
In a NTSC system, a color television signal is composed of a luminance signal and a chrominance signal separated by frequency division and amplitude modulated by a subcarrier of 3.58 MHz. In such a system, a cross color disturbance or a dot disturbance is often generaed due to mixing of the luminance signal with the chrominance signal during the time these signals are reproduced. Further, in the case where a color television signal is recorded on a recording medium by frequency modulation, moire patterns are liable to be generated on a reproduced image since the luminance signal is superimposed on a high frequency part of the chrominance signal and the modulation index is high when the instant frequency of the frequency modulation is close to the frequency of the chrominance signal.
In view of this problem a new color television signal processing system has been proposed which makes no use of the chrominance subcarrier and is free from the disadvantages described above. In this new signal processing system, two kinds of chrominance signals, such as the chrominance difference signals (R - Y) and (B - Y), are alternatively selected for each line, the time scale thereof is compressed and the signals are superposed on the luminance signal in such a manner that the compressed signal is located in the horizontal blanking period. When the transmitted or recorded signal is reproduced, the compressed chrominance signal is expanded in time scale and converted into a line sequential chrominance signal.
Furthermore, in the new system a delay line is provided for delaying one horizontal scanning time, and the shortage lines of each chrominance signal are complemented in order to obtain two kinds of original chrominance difference signals. Finally, an approximated NTSC signal is reproduced from the two kinds of chrominance signals and a luminance signal is obtained by conventional techniques.
In the above new signal processing system, the time compression and expansion technique is carried out as follows. The chrominance signal is sequentially sampled by a low frequency clock signal and applied to an analogue memory device, such as a charge transfer device or a capacitor memory. The sampled signals stored in the analogue memory are read out by a high frequency clock signal. Consequently, the output signal of the analogue memory becomes a time compressed chrominance signal.
The time expansion is carried out in a manner opposite to of the time compression, that is, the time compressed signal is sampled and applied to a charge transfer element by a high frequency clock signal, and when the sampled signals stored in the charge transfer element are read out by a low frequency clock signal, a time expanded signal is obtained.
Now, in the course of time compression and expansion, if there is a time shift between the time to be sampled in the time compression and the time a the high frequency clock signal in the time expansion, the time expanded chrominance signal will have an aberation based on the product of the time compression ratio with the time shift.
As a result, a time shift is created between the luminance signal and the chrominance signal.
In order to eliminate the above defect, a reference signal, such as a sinusoidal signal, is inserted in the horizontal synchronizing signal period before the time compressed chrominance signal, and the clock signal used for time expansion is produced in synchronism with the reference signal. Accordingly, even if the time divisional signal is affected by jitter in a recording and reproducing system, the time fluctuation of the time compressed chrominance signal is equal to that of the reference signal.
Therefore, the fluctuations of the luminance signal and the chrominance signal expanded by a clock signal derived from the reference signal are equal. Accordingly, the time shift between the above two signals is avoided.
However, if the reference signal should disappear due to a drop out in the recording medium or pulsating noise on the transmission line, and if the reset pulse should disappear or be generated at an incorrect time position depending on the above disappearance of the reference signal, the clock pulse signal used for time expansion cannot be obtained, or will be generated at a shifted time position.
This is a serious problem which limits the performance efficiency of the proposed color television signal processing system.